A depolarization signal (a small electrical impulse) is generated by most mammalian muscle tissue as such tissue contracts. Thus, the beating or contracting of a human heart is manifest by appropriate depolarization signals evidencing the contraction of the atria, referred to as the P-wave, and the contraction of the ventricles, referred to as the R-wave (or the QRS complex). The sequence of P-waves followed by R-waves thus comprises an electrogram or electrocardiogram signal that can be monitored by appropriate electrical circuits to indicate the status of the heart.
An implantable pacemaker includes sensing circuits that monitor the heart by looking for the occurrence of P-waves and/or R-waves, and pacing circuits that stimulate the heart with an appropriate electrical stimulation pulse in the event that a depolarization signal is not sensed within a prescribed time period. In this way, if the heart does not beat naturally within the prescribed time period, i.e., if the heart does not beat on its own so as to maintain a minimum heart rate as defined by the prescribed time period, then an electrical stimulation pulse is provided to force the heart muscle tissue to contract, thereby assuring that the prescribed minimum heart rate is maintained.
An implantable cardioverter-defibrillator (ICD) typically includes sensing and pacing circuits to provide electrical stimulation pulses aimed at responding to slow intrinsic (natural) cardiac rates or asystole (a non-beating heart). The pacing circuits may also provide appropriate electrical stimulation pulses, typically in a prescribed burst or pattern, aimed at terminating rapid intrinsic rates (tachyarrhythmias or tachycardias).
It is common in implantable devices such as pacemakers and ICD's to employ "refractory periods" during those time periods when the sensing circuits of the implantable device are inhibited or otherwise rendered ineffective. Refractory periods are necessary in an implantable device to prevent "over sensing," a phenomena wherein a natural or other event associated with a depolarization, such as the repolarization of cardiac tissue referred to as a T-wave, or an afterpotential remaining after a paced depolarization, is sensed and incorrectly assumed to be a natural depolarization. Thus, the refractory period defines a period of time immediately following a natural or paced depolarization during which all such natural or other events are blocked out and prevented from being sensed.
An ICD must sense intrinsic cardiac activity over a broad dynamic range. That is, cardiac activity ranging from very low amplitude ventricular fibrillation signals (evidenced by rapid depolarization signals having a magnitude of about 1 mV pk-pk or less), to higher amplitude ventricular tachycardia signals (evidenced by depolarization signals having a magnitude of up to 10 mV pk-pk), to normal ventricular and/or atrial depolarization signals (associated with slower intrinsic heart beats and also having a magnitude that may vary up to 10 mV pk-pk or higher), must all be sensed. Such sensing is made difficult because the sensitivity of the sensing circuits needed to sense one type of depolarization signal is not the same as is needed to sense another type of depolarization signal. Moreover, the relatively short refractory period needed to permit sensing very high rate tachyarrhythmias may be too short to effectively block or inhibit the sensing of afterpotentials or repolarizations or other commonly present signals associated with normal cardiac activity, evoked responses, or fibrillation. What is needed, therefore, is an ICD wherein both the sensitivity of the sensing circuits and the duration of the refractory period are automatically adjusted to optimal values depending upon the type of sensing function being carried out.